© 2015 American Chemical Society. We explore, from a theoretical perspective, the effect of particle size on the photocatalytic water splitting activity of TiO < inf > 2 < /inf > rutile (nano)particles by a combination of explicit quantum chemistry calculations on a hydroxylated rutile nanoparticle in a realistic solvation environment and a comparison with the calculated properties of bulk rutile (surfaces) from the literature. Specifically, we use density functional theory (DFT) and time-dependent DFT to calculate the nanoparticle thermodynamic driving force for the water splitting half-reactions and identify in the process the crucial role of self-trapping of the free charge carriers responsible for proton reduction and water oxidation.